Epithelial cells form coherent cell sheets called epithelia, which line the inner and outer surfaces of the body. Absorptive epithelial cells, such as those lining the intestine and urogenital tract, are bound together by junctions that give the epithelia sheet mechanical strength and also make it impermeable to small molecules. Typically, the epithelia forms a membraneous cell cover that serves to protect and cover other organs, and produces secretions and excretions, and is used in assimilation.
Rapid proliferation of epithelial cells is often the result of a mutation occurring in a group of founder cells, which can then replicate and divide at faster rates than non-mutated cells. Mutated clones of epithelial cells which proliferate and spread rapidly can form cancers, or malignant tumors, which grow at the expense of non-affected cells and ultimately destroy neighboring tissue. In contrast to tumors which are cancerous, benign tumors proliferate to excess, but do not disseminate through the body via the lymphatic or circulatory systems, and can often be easily treated.
However, rapid proliferation of epithelial cells does not always lead to formation of tumors. For instance, proliferative skin diseases, such as psoriasis, eczema, mycosis fungoides, and lichen planus, result in rapid sloughing off of the stratum corneum without forming either malignant or benign tumorous growths. Endometriosis is characterized by rapidly proliferating epithelial cells, although no tumors are formed. In addition, a decrease in the proliferation of epithelial cells, often resulting in the retardation of wound healing, can be indicative of diseased tissue, or excess use of steroids.
Optical methods involving excitation of tissue with light resonant with electronic transitions in epithelial cells are used to monitor several cell properties. These methods are potentially valuable because optical radiation can be easily delivered to tissue using fiber optic devices, and sensitive detection devices can be used to measure small signals, such as fluorescence and absorption, from cells.
In Alfano et al., U.S. Pat. No. 5,131,398, an optical method is disclosed which is said to be useful for distinguishing cancerous tissue from benign tumor tissue. This method uses the optical excitation of tissue with light centered at 300 nm, followed by detection of fluorescence. The ratio of the induced fluorescence at two different frequencies is then used to determine the presence and nature of the tumor.